The invention relates to LED light sources and in particular to adjusting the colour tone and colour temperature of LED light sources.
LED (Light Emitting Diode) lighting has become common in most diverse uses. The benefits of LED light sources include their energy efficiency and long service life, in particular. LED light sources may also be used inside cars as main and night-time lights. LED light sources are additionally well-suited for implementing security lighting of buildings, ships and other similar objects.
RGB (Red-Green-Blue) refers to the three primary colours of light, red, green and blue, out of which the other colours and white may be formed. A conventional single LED may only send single-colour (monochromatic) light, which may be one of these three primary colours. To create more colours, three LEDs may be used together for an RGB mix. An RGB LED is in principle three single-colour LEDs placed close to one another, often in the same package, and which are red, green and blue in colour. When all the LEDs of an RGB-LED emit at proportionately the same luminous intensity, and the right kind optics is used, the light emitted from an RGB-LED appears white to the human eye. The colour temperature of an RGB-LED may easily be changed by adjusting the emissive power of the differently coloured LEDs in the light source.
The adjusting methods most usually aim to affect the value of the forward current. A common method for adjusting the light intensity of an LED light source is to used pulse width modulation (PWM), that is, modulate the electric power fed to the light source. When the controlling takes place by pulse width modulation, the maximum value of the current fed to the LED is constant, but the average of the current is controlled by adjusting the length (on-time) of the current pulse in relation to the cycle time. Typically, a switching frequency higher than at least 200 Hz is used, so that the human eye cannot detect the flickering of the light. In the case of an RGB light source, the luminous intensity of each single colour LED or LED string is adjusted separately and each has its own constant-current regulator. The colour change and dimming of an RGB-LED light source therefore requires three PWM signals, each modulating the electric power feed to one LED string. Such a solution involves several drawbacks and problems. The power input of a light source is highly pulse-like and the current input varies in the range 0-300% of the nominal current of the light source. A change in the pulse ratio of each individual PWM signal affects both the colour tonecolour tone and the dimming. A full resolution for colour tonecolour tone only exists at 100% power (the combined pulse width of the PWM signals is 100%). When an LED is dimmed, the resolution of colour tones becomes worse and the colour tone changes at every step. The colour tones and intensities of adjacent LED strings vary according to the tolerances of the constant current regulators. The warming up of LED light fixtures makes the phenomenon worse.
Another known solution is the constant current reduction (CCR), in which the magnitude of an LEDs forward current is linearly adjusted. In the solution, each colour component in the LED string of each LED light source has its own adjustable current regulator, used to carry out the colour change and dimming of the light source by separately adjusting the current of the LED string of each colour component. This solution, too, involves several drawbacks and problems. The full resolution for a colour tone only exists at 100% of power. When an LED light source is dimmed, the resolution of colour tones becomes worse and the colour tone changes at every step. The resolution of a colour is weak even with the 100% power, because an LED have a specific minimum current below which the operation of the LE The intensity of an LED as a function of current is not linear, which makes it difficult to calculate the colour tone, and the colour tone changes with dimming partly due to this phenomenon. The microcontroller that controls a light source needs to check that the current of any colour component in the LED light source is not under the LEDs minimum current. If the current is about to go below the minimum, the colour component in question must be switched off or its current raised to the minimum level. The minimum intensity of white light is in an LED light source limited to three times the LED minimum current, because the LED of each colour must have at least a minimum current. The colour tones and intensities of adjacent LED strings vary according to the tolerances of their constant current regulators. The uneven warming up of an LED light source makes this phenomenon worse.